The present invention relates to an electrical resistor and more particularly to an electrical film resistor that may be trimmed to exhibit a predetermined resistance.
In order to help reduce the size of electronic products to more easily provide connections for components, electronic elements are often formed on a board, or substrate, rather than being made as discrete components and applied to the board. Resistors are often formed in such a manner by first making an electrically non-conductive substrate. A layer of an electrically resistive material and at least two electrically conductive terminals are applied to the substrate.
Electrical current may flow between the terminals through the electrically resistive film. The level of resistance between the terminals is determined by the physical geometry of the film between the terminals.
The geometry of the film between the two terminals is often changed by "trimming," or removing, selected portions of the film from the substrate. A laser may be used, for example, to make a "kerf," or cut, in the film and necessarily change the pattern of current flow through the film on the substrate.
Many applications, such as in hearing aids, require the manufacturer of the resistor to gradually vary the resistance between the terminals after the film has been applied to the substrate. Many presently available methods for trimming such film resistors, however, are not ideally suited for trimming over wide resistance ranges. As a result, the size of the substrate and film is larger than may otherwise be necessary.
In addition, many currently available film resistors exhibit resistance "drift." Thus, after the film resistor has been trimmed, the electrical resistance that it exhibits may, for example, vary over time or vary as a result of being exposed to different ambient temperatures. Such instabilitiy in the resistance may impair the performance of the electrical device incorporating the film resistor. Alternatively, the design of the electrical device may have to be larger and more complex than otherwise necessary in order to compensate for the drift that the film resistor exhibits.